Ask Mr. Smarty Plants

Your issues with growing plants under trees and dealing with the tree roots are a big challenge for many gardeners. Colorado State University Extension have put a very good article online about tree roots. It is by J.M. Sillick and W.R. Jacobi and is entitled “Healthy Roots and Healthy Trees.” Some of the key points are:
• Most tree roots are located in the top 6 to 24 inches of the soil and occupy an area two to four times the diameter of the tree crown. Roots grow where water, minerals and oxygen are found in the soil. Because the greatest supplies of these materials are usually located in the surface layer of soil, the largest concentration of feeder roots exists in this zone.
• Large roots and small feeder roots occupy a large area under ground. Typically, the root system of a tree extends outward past the dripline, two to four times the diameter of the average tree's crown.
• Roots obtain water, oxygen and minerals from soil. They do not grow toward anything or in any particular direction.
• Soil compaction, change in soil depth and improper watering can injure roots, increasing stress and susceptibility to disease and insects.
The Extension people say that changes in soil depth around trees can cause injury to root systems. The addition of only 4 to 6 inches of soil over a root zone drastically reduces the amount of oxygen and water available to the roots.  So cutting the roots out or adding additional soil on top of the roots is not a recommended solution.

Even though your trees are 10-15 feet apart, once they grow together they will focus their growth to the outward facing areas where the branches have the most sun exposure. More than likely they will fare just fine. If you are concerned, an arborist can give you a more detailed recommendation.

The best suggestion is to plant a low, native groundcover that tolerates dry shade and compacted soil under the trees or replace the old landscape plants with a mulch layer and create some new planting beds away from the ash trees where the roots will not interfere and there is more sunlight.  You should put your new planting beds two to four times the diameter of the mature ash crown.
The Ladybird Johnson Wildflower Center has a list of recommended native plants for your state in our plant database page. There are 173 plants to consider in the Minnesota Recommended list.

If you do decide to plant under the ash trees, Theresa Rooney of the University of Minnesota Master Gardeners has put together an information sheet with suggestions for planting in dry soil for shade or under trees. She gardens under a 100-year old elm tree and says that locations in dry shade are some of the most difficult to grow plants because of the competition for water, light and nutrients.

To compile a list of native plants for planting under the ash trees, go to our Native Plant Database. Under Combination Search, select the following categories: State – Minnesota, Habit – shrubs, and then search again for herbs (for herbaceous), Duration – perennial, Light Requirement – shade, and Soil Moisture - dry. You can further narrow down the list of potential plants by indicating whether you prefer a deciduous or evergreen plants, specific blooming times or blooming colors.

Some dry shade shrubs that grow in Minnesota that are in our native plant database include:
Amelanchier alnifolia (Saskatoon serviceberry)
Arctostaphylos uva-ursi (kinnikinnick)
Ceanothus americanus (New Jersey tea)
Diervilla lonicera (northern bush honeysuckle)
Elaeagnus commutata (silverberry)
Rhus aromatica (fragrant sumac)
Shepherdia canadensis (russet buffaloberry)
Symphoricarpos orbiculatus (coralberry)
Viburnum rafinesquianum (downy arrowwood)
And some native perennials for dry shade in Minnesota include:
Astragalus crassicarpus (groundplum milkvetch)
Geum canadense (white avens)
Lobelia spicata (palespike lobelia)
Maianthemum stellatum (starry false lily of the valley)
Mitchella repens (partridgeberry)
Polygonatum biflorum (smooth Solomon’s seal)
Pteridium aquilinum (western bracken fern)